Three republicans who secretly returned to Ireland after going on the run in Colombia, where they were convicted of training Marxist rebels, are unlikely to face extradition, legal experts said yesterday.

Known as the Colombia Three, Niall Connolly, Martin McCauley and James Monaghan were arrested in Colombia in 2001 on suspicion of training Farc guerrillas in the art of urban warfare.

But the bizarre saga - in which the men spent three years in a Bogotá prison before being acquitted, then jumped bail and went on the run through Latin America for eight months when their acquittal was overturned - is now causing a major headache for the Irish government.

After entering Ireland secretly, the three announced their arrival on national television last Friday, sparking fury among unionists.

The Ulster Unionist party said yesterday that unless they were extradited, Ireland was harbouring terrorists and shirking its duty in the international war on terror.

But Sinn Féin warned that the men - who said they went to Colombia to learn about its peace process - should not be extradited to a country with a poor record on human rights.

The Taoiseach, Bertie Ahern, yesterday admitted that the men's return had caused "enormous controversy and, in many quarters, anger". He said that their return had not been part of a political deal linked to the IRA announcement that its armed struggle was over.

The Colombian vice-president, Francisco Santos Calderón, yesterday insisted that the men must be extradited or made to serve their 17-year sentence in Ireland.

But Ireland has no extradition treaty with Colombia and legal experts said the lack of a treaty, coupled with concerns about poor human rights and prison security in Colombia, made the possibility remote.

Remy Farrell, a Dublin lawyer and expert in extradition law, said a decision to extradite could take many years and involve tough legal hurdles. The risk of the men being killed in prison in Colombia could be cited as a reason to refuse extradition, he added.